Hydrothermal Synthesis and Structure of a Dinuclear Molybdenum(III) Hydroxy Squarate with a Mo-Mo Bond

The reaction of molybdenum(II) and chromium(II) acetates with squaric acid in degassed and deionized water under hydrothermal conditions at 150 degrees C is described. The products have been formulated as M-2(mu-OH)(2) (mu-C4O4)(2)(H2O)(4)center dot 2H(2)O, where M = Cr (1) and Mo (2), based on combustion elemental analysis, infrared spectroscopy, magic angle spinning (MAS) solid-state carbon-13 nuclear magnetic resonance (NMR), and single-crystal X-ray diffraction. The edge-shared bioctahedral structures involve doubly bridging hydroxide ligands and mu-squarate ligands. The chromium compound lacks a direct metal-metalbonding interaction, while in contrast the molybdenum compound contains a Mo-Mo bond [2.491(2) angstrom]. The nature of the Mo-Mo-bonding interaction is compared with that of other similar d(3)-d(3) dimers.